1. Field of the Invention
The present invention is directed generally to rapid thermal processing devices and, more particularly, to an apparatus used in the fabrication of solid state devices to uniformly disburse gases and distortion-free radiant energy.
2. Description of the Background
In the fabrication of semi-conductor wafers, the deposition of a film on a surface of the wafer is a common and necessary step. The film is typically a semi-conductor, a conductor, or a dielectric. It is well known in the prior art that film deposition occurs more readily on a hot surface than on a cold surface. As a result, it is necessary to heat the surface of the wafer to induce film deposition. Wafers are typically heated and processed either by conventional batch furnace processing or by rapid thermal processing ("RTP").
RTP is an alternative to conventional batch furnace processing and is characterized by short processing times and rapid thermal rise and fall rates. An RTP process step typically lasts between several seconds and 15 minutes, with thermal rise rates typically between 100 and 500.degree. C. per second, and reaching temperatures of 1200.degree. C.
RTP has applications in the fabrication of very large scale integrated ("VLSI") circuits and ultra large scale integrated ("ULSI") circuits. In particular, RTP is used in the fabrication steps of thermal oxidation, thermal nitridation, dopant diffusion, thermal annealing, refractory metal silicide formation, and chemical vapor deposition ("CVD"). CVD may be used to form semi-conductive, conductive, and dielectric films. The design of RTP reactors is well known in the prior art, as disclosed, for example, in U.S. Pat. No. 5,446,825, issued to Moslehi et al., and U.S. Pat. No. 5,444,217, issued to Moore et al. An RTP reactor typically comprises a reaction chamber, a wafer handling system, a gas dispersion apparatus, a heat source, a temperature control system, and a gas control system.
The heat source is often high power lamps which drive chemical reactions in the reaction chamber and heat the wafer, thereby inducing film deposition on the surface of the wafer. The gas dispersion apparatus introduces gases into the reaction chamber so that chemical reactions can take place and films can be deposited on the surface of the wafer. Many types of gas dispersion apparatus are known, and one or more may be located below the wafer, to the side of the wafer, or above the wafer.
CVD process steps require both uniform gas distribution and uniform wafer temperature. If the gases are not distributed evenly over the surface of the wafer, the film will not be deposited evenly. That is in contrast to reactive processes, such as oxidation, which are not as sensitive to the distribution of the gases. That is because the gases in a reactive process are not deposited on the surface of a wafer, but rather react with the surface of the wafer, and therefore the process is self-limiting.
CVD process steps are also dependent on temperature, and if the surface of the wafer is not a uniform temperature, the film will not be deposited in a uniform manner. Furthermore, uneven heating of the wafer can cause slip dislocations, which are fractures in the crystal lattice, that may lead to a device failure.
One type of gas dispersion apparatus used for CVD process steps is known as a "showerhead." Showerheads are located above the wafer, have a generally flat bottom surface with a plurality of gas ports therein, and provide for a generally uniform distribution of gas over the surface of the wafer. Showerheads are made from transparent materials which do not absorb light, such as quartz.
To provide uniform heat to the surface of the wafer, heating lamps are located above the showerhead and separated from the reaction chamber by a transparent window. The use of both single and multiple lamps is known, as disclosed in U.S. Pat. No. 5,444,217, issued to Moore et al. The energy generated by the lamps is intended to travel through both the transparent window and the transparent showerhead, and be absorbed by the surface of the wafer.
It is well know in the prior art, however, that the temperature across a wafer is usually not uniform. One cause of nonuniform heating of a wafer is light distortion caused when light passes through the showerhead. As is well know in the prior art that film deposition occurs more readily on a hot surface than on a cold surface, and although quartz showerheads are very transparent, they still absorb some light and become hot. As a result, film depositions occur on showerheads, and these depositions absorb light, becoming hotter and inducing more film deposition. The result is a build up of film on the showerhead, which in turn blocks and distorts the light passing through the showerhead. This distortion causes uneven heating of the surface of the wafer and results in uneven film deposition, uneven film thickness, and can lead to defects in the wafer. In addition, film deposition on the showerhead may begin to flake, sending particulate matter into the reaction chamber and contaminating the wafer. To minimize those effects, the showerhead must be cleaned or replaced regularly.
Moving the showerhead away from the lamps, such as to the side of the reaction chamber, reduces the temperature of the showerhead but results in a less uniform distribution of gases, which is unacceptable in many applications. Although some other solutions of the light distortion problem have been proposed, such as the use of complex optics or special quartz showerheads, none of the solutions satisfactorily address the problem. Thus, a need exists for a device which both uniformly disperses gas and does not distort the light that is used for heating the surface of the wafer.